Sensors are used to detect events such as a glass break, motion, asset movement, temperature and impact/shock. These sensors can be used as a standalone device or in combination with a security system. A security system includes a life, safety, and property protection system. The sensors communicate with a control panel when the sensor detects an event.
Motion detectors contain a lens array or window/mirror array system which focuses Infrared (IR) energy produced by a human onto a pyroelectric sensor which converts the changes in IR energy reaching it into electrical signals. The energy produced by human body temperatures is in the range of 5 μM to 15 μM, also known as far IR. The lens array in one style of a motion detector and the window in another style containing a mirror array must allow far IR energy to pass in order for the energy to be focused on the pyroelectric sensor If the lens or window is tampered with in such a way to prohibit passage of far IR energy, the motion detector will fail to detect an intruder.
Motion sensors or detectors can be intentionally tampered by an intruder. For example, the intruder can mask the window or lens of the detector by a coating, such as paints. Masking of the detector prevents the IR energy from reaching the pyroelectric sensor of the detector, thereby undermining the detection functionality of detecting motion within a protected area. Generally, spraying or brushing a coating or film on the window or lens of the sensor blocking the infrared signal can mask a detector, such as a passive infrared sensor (PIR).
In light of the foregoing problems, many motion detectors are designed to include systems to detect intentional masking or blocking of the sensing element of the sensor. These systems are commonly referred to as anti-mask systems. For example, these motion detectors employ active near infrared sensors (wavelengths just beyond the visible spectrum typically around 900 nM) as their basic instrument for anti-masking. Honeywell “Viewguard PIR AM” is an example of such motion detectors using active near infrared sensing system, and is market available. In an alternate embodiment, these motion detectors may use a microwave sensing system as anti-mask systems. Honeywell DT7550 is an example of such motion detectors using microwave sensing system, and is market available.
However, these detectors have inherent weakness to paints applied with brushes tenderly or spray cans. The act of a spray application only causes a slight transient change in the active near infrared sensor signal. However, this transient change can be caused by other events not associated with masking, such as an electrical spike or a pass of an object. The act of applying clear paints via a brush generally causes significant swings in the active near infrared sensor signal. However, a fluttering moth or a feather duster can cause the same swings as the brush. Thus, detection of only transient change or swings in the detector signal is not sufficient and sometimes not reliable to detect masking, because a false alarm indicating tampering may be issued.
Furthermore, so-called clear sprayed on materials, such as hair spay or clear paint, are inherently difficult to detect. Other difficult materials are clear adhesives, such as clear tapes, applied very smoothly to the lens without wrinkles or air bubbles, which are nearly impossible to detect after the application. Clear paints and clear adhesives are transparent to visible and near IR (Infrared) energy but block far IR energy. Thus, the window of the detector covered in clear paint or adhesive blocks the thermal energy from an intruder from reaching the pyroelectric sensor, which renders the detector blind.
Therefore, it would be very advantageous to provide a motion detector and a method having an anti-mask mechanism capable of effectively and reliably detecting any possible intentional masking of the detector.